1. Field of the Invention
The present invention relates to a fixing device for use in fixing unfixed toner particles onto a recording medium, and an image forming apparatus using the fixing device.
2. Discussion of the Background
Image forming apparatuses using electrophotography include a fixing device to fix an unfixed toner image transferred onto a recording medium. Such a fixing device typically adopts a heat roller system.
A fixing device adopting a heat roller system includes a fixing roller containing a heat source such as a halogen heater therein and a pressure roller. The pressure roller is in contact under pressure with the fixing roller, thereby forming a nip therebetween. The fixing device adopting a heat roller system fixes unfixed toner images onto a recording medium while the recording medium is passing through the nip. The fixing roller used in the fixing device adopting a heat roller system is formed of a core made of, for example, metals, on which an elastic layer (i.e., a heat insulating layer) and a release layer are formed. This type of fixing device has a relatively large thermal capacity. It naturally takes a long time to raise the temperature of such a roller by heat from room temperature to a predetermined temperature.
Reduction in the time needed to be taken to raise the temperature (hereinafter referred to as rise time) is essential for energy saving. By reducing the thickness of the core and the elastic layer of such a roller applying heat to unfixed toner, a relatively low thermal capacity can be imparted to the roller, resulting in shortening the rise time of the roller. The roller applying heat to unfixed toner refers to a roller directly heated by a heater. However, a background roller applying heat to unfixed toner typically contains a heat source therein. Therefore it takes a long time before the heat is conveyed to the surface of the roller. Especially, when the roller is used for fixing color images, the elastic layer thereof has to be sufficiently thick to obtain quality images. Therefore, such a roller has a long rise time. In addition, it also takes a long time to raise the temperature of such a roller to a predetermined temperature again after the roller is deprived of heat by a recording medium passing through the nip. Therefore, when recording media consecutively pass through the nip, the fixing ability of a fixing device using such a roller deteriorates because the temperature of the heat roller falls.
Published unexamined Japanese Patent Application No. JP 2002-40855 discloses a fixing device including a system in which a heat source is disposed outside the fixing member to directly apply heat to the surface thereof. In this system, the fixing roller has a substantially short rise time because heat is applied only to the surface layer to be heated and in addition has a quick response to the temperature fall caused by recording media passing through the nip. However, the heat conveyed to the surface layer in fact diffuses into a portion not to be heated such as the inside of the fixing roller and the pressure roller. Thus, the actual rise time of the fixing roller is longer than its theoretical time.
It is possible to shorten the rise time of a fixing roller by forming a heat insulating layer under the surface layer of the heating roller to improve thermal insulation properties thereof. As materials having good thermal insulation properties, there are materials containing a gas such as air having a low specific thermal conductivity therein. As disclosed in JP 2000-206815, foamed silicone rubber is widely used. Foamed silicone has a low specific thermal conductivity and good thermal insulation properties because of air contained therein. However, the air contained in the foamed silicone is compressed at the nip, resulting in reduction in the content ratio of the air. Thereby, the specific thermal conductivity of the foamed silicone becomes high. In addition, foamed silicone is easily transformed by compression. Thus, it is unsuitable to apply a high plane pressure to foamed silicone. Further, foamed silicone tends to deteriorate over time with repetitive transformation. Furthermore, foamed silicone has another drawback in that, since the rotation radius of a foamed silicone roller varies under compression, such a roller is not suitable as a driving roller to convey recording media, etc., at a constant speed.
JP 2002-40855 exemplifies porous ceramics and porous resin as a thermal insulation material having a high rigidity with little transformation. JP 2000-275996 and JP 2001-65544 disclose a binder mixed with hollow particles. However, these materials have insufficient thermal insulation properties.
Further, JP 2001-343850 discloses a fixing roller including a heat insulating layer formed of accumulated layers including resin films having holes. However, this structure has a drawback in that it is difficult to appropriately position the holes in adjacent film layers and to form a heat insulating layer having a large void ratio.